Air gap formation method for reducing undesired capacitive coupling between interconnects in an integrated circuit device

ABSTRACT

An air gap structure and formation method for substantially reducing the undesired capacitance between adjacent interconnects, metal lines or other features in an integrated circuit device is disclosed. The air gap extends above, and may also additionally extend below, the interconnects desired to be isolated thus minimizing fringing fields between the lines. The integrated air gap structure and formation method can be utilized in conjunction with either damascene or conventional integrated circuit metallization schemes. Also, multiple levels of the integrated air gap structure can be fabricated to accommodate multiple metal levels while always ensuring that physical dielectric layer support is provided to the device structure underlying the interconnects.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and is a divisional of parentapplication Ser. No. 10/295,062, filed Nov. 15, 2002, now U.S. Pat. No.6,917,109 which is hereby incorporated by reference. The parentapplication is related to the following applications: Air Gap for DualDamascene applications Ser. No. 10/295,795, filed Oct. 15, 2002, and AirGap for Tungsten/Aluminum Plug applications Ser. No. 10,295,080 filedOct. 15, 2002. The aforementioned are hereby incorporated by referenceas if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates, in general, to the field of integratedcircuit (“IC”) device structures and methods of forming the same. Moreparticularly, the present invention relates to an air gap structure andformation method for reducing undesired capacitive coupling betweeninterconnects and/or other elements in an integrated circuit device.

BACKGROUND OF THE INVENTION

As integrated circuit transistor densities increase, and feature sizesshrink, capacitive coupling between adjacent interconnects, metal linesor other elements also increases. The increased capacitive couplingresults in increased parasitic capacitance, which undesirably slowscircuit speeds and negatively impacts overall device performance.

Current attempts to improve electrical isolation in high densityintegrated circuits involve the implementation of low K dielectricmaterials such as hydrogen silsesquioxane (HSQ), SiLK™ (a trademark ofThe Dow Chemical Company) resin, Black Diamond™ (a trademark of AppliedMaterials company) low K film, Coral™ (a trademark of Novellus SystemInc.) carbonaceous oxide film and several other exotic materials. Whilethese materials have a relatively low dielectric constant, they are notnormally used in semiconductor manufacturing and therefore increasemanufacturing complexity and costs. Much work remains to effectivelyintegrate these materials into conventional semiconductor manufacturingprocesses.

Some disadvantages of current low K materials include incompatiblethermal coefficient of expansion, low mechanical strength and poorthermal diffusivity.

Another manner of improving electrical isolation between interconnectsis to use an integrated air gap structure because of the extremely lowdielectric constant of air. Previous attempts at air gap structures werehard to manufacture and also did not completely isolate adjacent metallines due to fringing fields above and below the air gap itself.

For example, U.S. Pat. No. 6,177,329 to Pang (and particularly at col.7, ll. 46+) illustrates one conventional approach in which an additionalmask is used to pattern the underlying layers to form the air gaps. Thisis both inefficient and imprecise for extremely small geometries. U.S.Pat. No. 5,847,439 to Reinberg illustrates another approach in which acombination of a low melting point dielectric, photoresist, a heat cycleand surface tension interact to form a void between two adjacent metallines. This technique is clearly not suitable for precise control of airgap sizes, and is further disadvantageous because it cannot be used toform gaps which extend above a metal line. The latter may be desirablein some applications. Finally, U.S. Pat. No. 5,949,143 to Bang depicts arather complex process in which a small opening is made in an etch stoplayer and then a selective isotropic etch is used to remove dielectricbetween two metal lines.

Clearly, while portions of the aforementioned references are useful informing air gap structures, and could be used in many applications,their overall approach is not optimal from a manufacturing perspective.

What is desired, therefore, is an easily manufacturable integrated airgap structure that substantially electrically isolates adjacentinterconnects, metal lines or other IC elements.

SUMMARY OF THE INVENTION

In accordance with the structure and method disclosed herein, a firstmethod for forming a device having an air gap structure includes forminga device layer, which can include first level metal, capacitors,transistors, or other integrated circuit devices, as well as previouslyformed air gap structures fabricated according to the method of thepresent invention. A dual damascene structure with a plurality dualdamascene opening is formed over the device layer, including first andsecond patterned dielectric layers. A copper or other conductive layeris formed to fill the dual damascene opening. An adjustable-depth trenchis formed between the conductive pattern at least down to the surface ofthe device layer. The dual damascene structure itself is used as a hardmask in the etching of the trench. Finally, a third dielectric layer isformed onto the trench to form at least one air gap, the air gapoptionally extending above the top surface of the dual damascenestructure. If desired, the depth of the trench can be extended below thesurface of the device layer.

A second method for forming an air gap structure in an integratedcircuit according to the present invention includes forming aninterconnect structure on the device layer including, for example, anpatterned aluminum or aluminum alloy (conductive aluminum with orwithout minor amounts of another element or elements) conductive layeroverlaying a tungsten conductive plug layer.

An adjustable-depth trench is formed between the patterned interconnectstructure at least down to the surface of the device layer. A dielectriclayer is formed over the trench to form an air gap therein, the air gapoptionally extending above the top surface of the interconnectstructure. If desired, the depth of the trench can be etched to extendbelow the surface of the device layer.

A third method for forming an air gap structure for an integratedcircuit according to the present invention includes forming aninterconnect structure on the device layer including an aluminum alloyinterconnect layer overlaying an aluminum alloy plug layer. Theconductive plug layer and interconnect layer can be formedsimultaneously, thus eliminating at least two processing steps ascompared to the second method of the present invention. Anadjustable-depth trench is formed between the patterned interconnectstructure at least down to the surface of the device layer. A dielectriclayer is formed on the trench to form an air gap therein, the air gapoptionally extending above the top surface of the interconnectstructure. If desired, the depth of the trench can be etched to extendbelow the surface of the device layer.

It is an advantage of the present invention that the low dielectricconstant of air is used to provide maximum electrical isolation byextending the air gap both below and above the adjacent isolatedinterconnects, or metal lines, while still ensuring that physicaldielectric support is provided beneath the interconnects themselves.

It is a further advantage of the present invention that the air gapisolation structure is readily manufacturable and compatible withexisting semiconductor manufacturing techniques.

It is a still further advantage of the present invention that exotic lowK dielectric materials need not be used, thus saving costs andminimizing manufacturing complexity.

It is a still further advantage of the present invention that theexistence of the air gaps is to release most of the system stressgenerated by subsequent thermal treatments.

It is a still further advantage of the present invention that thenetwork structure using conventional dielectric layers encompassing theinterconnects provides good thermal dissipation.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other features and objects of the presentinvention and the manner of attaining them will become more apparent andthe invention itself will be best understood by reference to thefollowing description of a preferred embodiment taken in conjunctionwith the accompanying drawings, wherein:

FIGS. 1-12 are cross-sectional views of sequential integrated circuitprocessing steps for forming an air gap isolation structure according toa first embodiment of the present invention, using one of severalacceptable dual-damascene metal interconnect processes;

FIG. 13 is a cross-sectional view of a resulting air gap isolationstructure according to the present invention, accommodating the use ofmultiple levels of a dual-damascene metal interconnect process;

FIGS. 14-24 are cross-sectional views of sequential integrated circuitprocessing steps for forming an air gap isolation structure according toa second embodiment of the present invention using one of severalacceptable conventional metal interconnect processes;

FIGS. 25-33 are cross-sectional views of sequential integrated circuitprocessing steps for forming an air gap isolation structure according toa third embodiment of the present invention using a conventional metalinterconnect process;

FIG. 34 is a cross-sectional view of an air gap isolation structureaccording to the second/third embodiments of the present invention,accommodating the use of multiple levels of a conventional metalinterconnect process; and

FIGS. 35-38 are cross-sectional views of sequential integrated circuitprocessing steps for forming an air gap isolation structure according toa fourth embodiment of the present invention, which is a variant of thefirst embodiment in which an etch stop layer between a line dielectricand a via dielectric is eliminated to further reduce the effectivedielectric constant of the inter-metal dielectric layer; and

FIGS. 39-40 are cross-sectional views of sequential integrated circuitprocessing steps for forming an air gap isolation structure according toa fifth embodiment of the present invention, which is a variant of thefirst embodiment in which a first etch is performed only as far as afirst etch stop layer.

DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment

Referring generally now to FIGS. 1-13, a method for forming anintegrated circuit device having an air gap structure is shown for adual damascene-type metal interconnect structure.

In FIG. 1, a device layer 10 is formed, which may be a simple siliconsubstrate and first-level metal, for example. The device layer 10 maynonetheless also include multiple levels of metal, transistors,capacitors, or other devices, including previously manufacturedintegrated air gap structures built according to the method of thepresent invention. Thus, device layer 10 is meant to represent thatportion of the previously formed integrated circuit device on which theair gap structure is to be built, but it is not limited to anyparticular form, structure or circuitry.

Similarly, as used herein, the terms “on” or “onto” or “above” when usedin connection with various thin film layers are merely intended todenote a physical spatial relationship, and not necessarily a directphysical or electrical contact. It will be understood therefore by thoseskilled in the art that in embodiments of the invention, a first layermay be “on” or “above” a second layer, even if there are otherintervening layers present.

In a first embodiment, a first etch stop layer 12 is formed on the uppersurface of the device layer 10. The etch stop layer 12 is ideally formedof silicon nitride (SiNx), silicon oxynitride (SiNxOy), silicon carbide(SiCx), or the like, and is deposited to a thickness of about 100 to1500 Angstroms using any of a number of known conventional mechanisms.The particular material for any application of course can be determinedby one skilled in the art by coordinating such selection with an etchchemistry/mechanism to be employed in a later etch operation. Thus, solong as such first etch stop layer is otherwise compatible with othermaterials and processes described herein, the present invention is notlimited to any particular material.

A first dielectric layer 14 (designated generally herein as a “via”dielectric layer because the body of a via contact is later formedtherein) is formed on etch stop layer 12. The first dielectric layer 14is ideally silicon dioxide or undoped silicate glass (USG) but can alsobe fluorinated silicate glass (FSG), or borophosphorus silicate glass(BPSG), phosphorus silicate glass (PSG), or the like and is deposited toa thickness of about 1000 to 10000 Angstroms using well-known processingtools. Moreover, first dielectric layer 14 can include combinationsand/or composites of individual thin film layers. Again, the particularformulation for this layer will depend on desired performancecharacteristics and process requirements, and thus a variety ofmaterials are expected to be suitable for such layer.

In FIG. 2, an additional second etch stop layer 16 is deposited onto theupper surface of via dielectric layer 14. As with etch stop layer 12,the particular composition of etch stop layer 16 is not critical, andcan be determined without undue experimentation by one skilled in theart based on the present teachings and objectives defined herein for theinventions presented.

A second dielectric layer 18 (designated generally herein as a “line”dielectric layer because portions of a conductive line are later formedtherein) is deposited onto the surface of etch stop layer 16. The linedielectric is also ideally silicon dioxide or a similar dielectric asvia dielectric layer 14 and is deposited to a thickness of about 1000 to10000 Angstroms. The selection of materials for this layer will again bea routine design choice based on lithographic and etching requirementsassociated with a particular manufacturing process.

A third etch stop and/or an anti-reflecting layer 20 is subsequentlydeposited on the line dielectric layer 18. Etch stop and/oranti-reflecting layer 20 is preferably SiNx, SiNxOy, silicon richedoxide (SRO), SiCx or the like and is deposited to a thickness of about100 to 500 Angstroms. As with the other etch stop layers, the particularmaterial for any application of course can be determined by one skilledin the art by coordinating such selection with an etchchemistry/mechanism to be employed in a later etch operation.

In general, the overall composition of the structure shown in FIG. 2 canbe constructed with conventional and well-known manufacturing equipmentsuitable for wafer processing operations. The particular selection ofmaterials for the thin film layers is directed primarily by concerns ofreliability, reproducibility and lithographic constraints in small scalegeometries, and so it is expected that a wide variety of combinationswill be suitable for use in the present invention.

In FIG. 3, a photoresist layer 22 is formed on third etch stop and/oranti-reflecting layer 20 to a thickness of about 1000 to 10000Angstroms. Photoresist layer 22 is patterned to form metal contacts or avia pattern 24A by any conventional photolithography process. Theparticular resist formulation and lithography process are again notmaterial to the present teachings, so any suitable combination may beemployed.

In FIG. 4, line dielectric layer 18 is anisotropically etched using viapattern 24A as a mask to form a metal contact or via opening 24B. Aconventional oxide etch such as reactive ion etch (RIE) can be used forthis step, which is terminated upon reaching first etch stop layer 12,or some other point before this. Other techniques will be apparent tothose skilled in the art. It should be noted, of course, that etch stoplayer 12 can also be removed in those areas (not shown) where it may bedesirable to make a conductive contact to some portion of device layerportion 10.

In FIG. 5, after removing the resist layer 22, another photoresist layer32 is processed to form a metal line pattern 24C by photolithographyprocesses. Again, the particular resist formulation and lithographyprocess for layer 32 are again not material to the present teachings, soany suitable combination may be employed.

In FIG. 6, both line dielectric layer 18 and via dielectric layer 14 areetched to form an opening 24D for subsequent processing of the dualdamascene structure. This etching operation is also done with aconventional etch such as reactive ion etch (RIE) can be used for thisstep, which is preferably terminated upon reaching second etch stoplayer 16. Other techniques will be apparent to those skilled in the art.Thus, both photoresist layer 32 and the patterned etch stop layer 16 actas a form of mask for this operation.

It should be noted that the upper portion of opening 24D serves as aninterconnect line while the bottom portion of opening 24D functions as aconductive pillar to the device portion. The result is a conductive line28 with a cross section in some areas that resembles a T-shape as seenin the Figures.

In FIG. 7, resist layer 32 is stripped using a conventional process anda composite copper barrier/seed layer (shown as a single integratedlayer 26 for simplicity) is deposited using conventional means. Thefirst portion of copper barrier/seed layer is a barrier layer selectedfrom a group of conductive materials that can prevent Cu from diffusinginto adjacent dielectric layers, such as Ta, TaN, TiN, TiW, WN, Mo, W,etc. These are examples known to the inventors at this time, and it ispossible of course that later developed materials unforeseen and as yetundiscovered may prove to be suitable for this purpose.

A seed layer portion of composite barrier/seed layer 26 is typically Cuor Cu alloy, again deposited using known means.

In a preferred embodiment, the copper barrier layer portion is depositedto a thickness of about 50 to 500 Angstroms, and the seed layer portionis deposited to a thickness of about 300 to 2000 Angstroms to formcombined layer 26. It will be understood by those skilled in the artthat these values are merely exemplary for the geometries describedtherein, and that the final values for any particular embodiment of theinvention may deviate from such figures.

In FIG. 8, opening 24D is then filled with a copper layer 28. Copper isdeposited to a thickness of about 2000 to 10000 Angstroms using anywell-known conventional tools, which preferably completely fills opening24D and provides an excess copper layer. It will be understood, ofcourse, that the deposition of this layer may be achieved in a singlestep, or multiple steps to provide a graded and/or composite copperlayer within opening 24D.

In FIG. 9, any excess copper on top of line dielectric 18 is removedpreferably using chemical-mechanical polishing (CMP) with a suitablepolish pad, slurry, recipe, etc. as is known to those skilled in theart. In self-limiting growth processes, this type of CMP operation maybe minimized or reduced. The above steps for defining the openings andforming the Cu lines within such openings 24D are merely an example ofthe preferred technique known to the inventors at this time, and it ispossible of course that later developed processes unforeseen and as yetundiscovered may prove to be suitable for such purposes.

In FIG. 10, a plurality of dual damascene metal conductive lines 28 forman interconnect structure 28′. Each dual damascene metal interconnectline 28 is isolated primarily at this point by a combination ofdielectric layers 14 and 18.

Other cross-sectional portions of a wafer are illustrated in FIG. 10A toshow some additional examples of structures/relationships that mayexist. For example, in some areas an conductive line 29 may not extenddown to device layer 10 (the most likely case for a metal line); inother areas 29′ the position of the via is not symmetric about the metalline. In other areas 29″ the via part of the dual damascene structuremay extend to the device portion 10 and may be contacted to thesubstrate. In other area 29′″, the metal line part of the dual damascenestructure is about the same width as that of via parts. Thus, a varietyof cross-sectional patterns will result. It will be understood by thoseskilled in the art that these are merely exemplary, and that otherportions of a wafer are likely to contain additional variants of thoseillustrated depending on interconnect/masking requirements.

As alluded to earlier, at least some of the conductive lines 28 may beincluded as part of a so-called “dummy” pattern so as to make theinterconnection patterns more uniform across the surface of a wafer.This also facilitates the manufacturing process because the resultingsurface is more uniform.

In FIG. 10B, a side perspective can be seen of another exemplaryconductive line 28 viewed lengthwise as it may be formed for anintegrated circuit. At individual points across the surface, a lowerportion of conductive line 28 extends (in some instances) as a type ofconductive pillar 11 to form an electrical contact at selective pointsto device layer 10. These conductive pillars are formed from acombination of material from conductive line 28 that is surrounded bydielectric material 14 for additional support.

In FIG. 11, dielectric layers 14 and 18 from FIG. 10 arepreferably-anisotropically etched using copper layer 28 as a hard mask.A conventional dielectric etch is used to form trenches 30 intodielectric layers 14 and 18. The form and depth of trenches 30 isadjustable and can extend down to the upper surface of the device layer10, or can be etched further to extend down below etch stop layer 12 andbelow the surface of device layer 10 (not shown in FIG. 11).

For reasons that are explained in more detail below, an anisotropic etch(or an etch type with reduced isotropic behavior) is preferred over a“wet” isotropic etch at this point, because it is desirable to leavesome small amount of dielectric on the sidewalls of interconnect 18,underneath the overhang areas as seen in FIG. 11. Of course, in somecases it may be desirable to remove such remaining material (from layer14) and replace it with another material (i.e., through another spin ondeposition/plasma deposition and subsequent etch. An isotropic etchcould then be used on layer 14. While this would require additionalprocessing steps, it is conceivable that the dielectric constant couldbe improved in this fashion, as well as reliability, yield etc. of theoverall process.

The depth of trenches 30 is preferably controlled through a timed etch,and it will be apparent to those skilled in the art that the duration ofsuch etch will be a function of the dielectric layer composition, theetch process chemistry, the thickness of layers 14, 18, etc., etc. Theetch time will thus vary from application to application, and can bedetermined with routine simulations and testings.

Alternatively it is possible instead to use either etch stop layer 12 tocontrol the end of the etch, and/or to provide yet another etch stoplayer (not shown) within layer 14 at any optimally determined etchdepth. In such instance, of course, layer 14 would be a composite layerdeposited in separate steps, and thus this option is not as attractivefrom a throughput perspective.

As noted above, a preferred approach uses copper conductive lines 28 asa mask, but it those skilled in the art will appreciate that anadditional masking step could be employed should it be necessary to makethe air gaps more narrow. Again, this is not optimal from a control andthroughput perspective, so it is probably not desirable except inlimited cases.

In contrast, in the present invention, it should be relatively simpleand easy to control the size of such air gaps both by controlling thespacing between the conductive lines 28, as well as tailoring thesize/shape of the top portion of the conductive line. This is true sincethe latter effectuate the hard mask used for etching dielectric layers14, 18 to form the air gaps.

In this respect, those skilled in the art will appreciate that shapesand sizes of the interconnect structures shown in the figures are onlyapproximate, and not intended to be to scale. Other variations areexpected to be beneficially employed in accordance with the presentteachings.

In FIG. 12, a copper barrier layer 44 such as SiNx, SiC, or the like isdeposited to a thickness of about 50 to 500 Angstroms. Again, these arematerials particularly suited for copper, and other compositions may beneeded for other types of conductive line metals. For some metals, ofcourse, a barrier layer may not be needed in the first place.

A silicon-dioxide dielectric layer, or the like 32 is then deposited toa thickness of about 2000 to 10000 Angstroms. Poor step coverage by thedeposition of dielectric layer 32, such as conventional plasma enhancedchemical vapor deposition (PECVD), results in the formation ofintra-metal line air gaps 34. In other words, the present inventionexploits the basically conformal growth nature of this type of processto intentionally form gaps between the metal lines. By controlling thedeposition parameters, and the thickness of the deposited layer, thesize, shape and height of air gaps 34 can be customized for anyparticular line interconnect geometry.

In lieu of a PECVD process, other similar techniques that arecharacterized by poor step coverage could be used to form air gaps 34.For example, a series of HDPCVD depositions could be used. As thoseskilled in the art will appreciate, the above are merely examples oftechniques for achieving poor step coverage that are known to theinventors at this time, and it is possible of course that laterdeveloped processes unforeseen and as yet undiscovered may prove to besuitable for such purposes.

As previously discussed, the inclusion of air gaps 34 provides superiorelectric isolation due to the low dielectric constant of air. The sizeand shape of air gaps 34 may also vary across the surface of a wafer, asillustrated generally in FIG. 12A. It can be seen in such picture thatthe width of any air gaps (W1 or W2) are not necessarily uniform acrossthe surface of the wafer, nor are they required to be for purposes ofthe present invention. It is simply desirable, of course, to ensure thatat least some air filled gap is provided between two adjacent signallines.

Thus, as seen in FIG. 12A, one useful benchmark is to consider therelative ratio of the airgap width (W1, W2) to an overall line spacing(WS1, WS2). In general, the closer W1/WS1 and W2/WS2 are to unity, thelower the capacitance, so it is preferable to maximize this value to theextent consistent with other processing requirements.

In addition, the height by which the air gaps 34 extend aboveinterconnect layer 28, or below such layer, is controlled both by thetrench sizing noted earlier, as well as the details of the conformaldielectric deposition noted earlier. Thus, they may also vary invertical size as seen in FIG. 12B, where two different heights (H1 andH2) are provided. Again it is understood that the height of any air gaps(H1 or H2) are not necessarily uniform across the surface of the wafer,nor are they required to be for purposes of the present invention.Nonetheless, for reasons well understood in the art, it is preferable(to the exent possible within available process constraints) to maximizesuch air gap heights (in relation to the height HL of the conductivelines 28) by extending them above and below an interconnect structure 28to reduce the capacitance between adjacent lines.

In summary, an inter-line interconnect structure as shown in FIG. 12typically includes a metal line 28, an conductive line sidewalldielectric portion 14′, a second dielectric filler 32, and air gap 34.The sidewall dielectric portion 14′ left underneath metal line 28provides structural support and additional process window margin whenthe present invention is used in small scale line width geometries.

Those skilled in the art will further appreciate that the above aremerely examples of what might be present in any section of the wafer,and that other air gap structures will inevitably result as part of anyconventional manufacturing process employing the present teachings.

As further noted, to reduce non-uniformities for such air gaps, dummymetal lines can be added to an interconnect pattern to ensure that nolarge flat spaces are left between adjacent conductive lines. Thus, forexample, in FIG. 12, for some instances across the surface of the wafer,the middle metal line 28 may be carrying an actual signal, and in otherinstances, a “dummy” metal line 28 may be simply added so as to create auniform capacitance everywhere for the metal lines adjacent thereto.

In FIG. 13, a composite drawing is shown of two dual damascenestructures fabricated in sequence according to the method of the presentinvention. A device layer 10 includes a silicon substrate and a portionof first level of metal interconnect 28 extends herein as well. In afirst level of interconnect structure according to the presentinvention, copper metal layer 28 and air gaps 34 are shown extending inand to the top of dielectric layers 14 and 18. Note that air gaps 34 areshown to extend below the level of the upper surface of the device layer10 as well as above the upper surface of metal lines 28, thus providingthe maximum electrical isolation between adjacent metal structures.

Also shown in FIG. 13 is a second level of metal interconnect thatincludes an additional metal interconnect 38 and a dielectric layer 36with air gaps 40 and 42. Air gap 40 provides intra-level metal isolationand extends to the surface of device layer including layers 10, 14, and18, as well as above the upper surface of metal lines 38. Air gaps 42extend below the surface of the device layer including layers 10, 14,and 18, and thus provide electrical intra-metal isolation for both metallayers 38 and 28.

Furthermore it will be apparent that this overall process could berepeated as needed to form additional interconnect layers, and thepresent invention is by no means limited to any particular number ofsuch layers.

Another important observation about the present invention that can begleaned from FIG. 13, is that in some instances an air gap for a secondlevel interconnect may be formed on top of a first level interconnect.In other instances a single air gap can be extended in height so that itserves to reduce capacitance for more than one interconnect layer. Forexample, the air gap 42 shown in the middle of FIG. 13 serves as an airgap for two separate metal interconnect levels; this same principlecould be extended as needed for additional levels. Thus by appropriate“stacking” and arrangement of interconnect layers, a single air gap canbe formed between adjacently located conductive lines in more than onelayer of metal.

As illustrated herein, the dielectric material 14 underneath theconductive lines further functions to provide some measure of structuralsupport for the latter. This feature can be enhanced or reduced in otherembodiments by structural variations so that more or less dielectric isleft on the sidewalls, or under the top portions of the conductivelines. The dielectric also functions as a heat dissipator, and furtherreduces electromigration. Accordingly, the amount of dielectric left onthe sidewalls can be tailored for any particular environment, so that itmight be used extensively in some applications (thicker layers), and notused in others (thin layers, or no layers at all).

Second Embodiment

Referring generally now to FIGS. 14-24, a method for forming anintegrated circuit device having at least one air gap structure is shownfor a conventional metal interconnect structure of the type havingaluminum alloy metal interconnect layers and tungsten metal plugs.Except where otherwise noted, like numerals are intended to representlike structures and materials already identified in connection withFIGS. 1-13.

In FIG. 14, a device layer 10 is formed as before.

A contact/via dielectric layer 14 is formed on device layer 10. Asbefore, dielectric layer 14 is ideally silicon dioxide but can also beUSG, FSG, PSG, BPSG, or the like and is deposited to a thickness ofabout 1000 to 10000 Angstroms. It will be understood, of course, thatlayer 14 may be comprised of a combination of layers, and formed in morethan one processing step, but for purposes of the present discussion, itwill be referred to as a single layer.

In FIG. 15, a photoresist layer 22 is formed on dielectric layer 14 to athickness of about 1000 to 10000 Angstroms. Photoresist layer 22 ispatterned to form metal contact or via pattern 56A by photolithographyprocesses as before.

In FIG. 16, as noted before, openings 56B are etched into thecontact/via dielectric layer 52 in a similar fashion to that alreadydescribed for FIG. 4.

In FIG. 17, resist layer 22 is stripped and a tungsten barrier layer 92(such as Ti/TiN, Ta, TaN etc.) is deposited on the surface of dielectriclayer 52 and in openings 56. Again, these are merely examples of thoseknown at this time to be particularly suited for Tungsten, and othercompositions may be needed for other types of conductive line metals.For some metals, of course, a barrier layer may not be needed in thefirst place.

A layer of Tungsten 58 is then preferably deposited to a thickness ofabout 500 to 8000 Angstroms, which completely fills openings 56. Again,for other processes, materials other than Tungsten may be more suitable.

In FIG. 18, any excess tungsten is removed using tungsten CMP ortungsten etch back, which results in a structure that includesdielectric layer 14 and tungsten metal plugs 58. In self-limiting growthprocesses, this type of CMP operation may be minimized or reduced.

In FIG. 19, an aluminum alloy (or the like) interconnect layer 60 isdeposited on combined metal plug/dielectric layer 14/58 to a thicknessof about 2000 to 10000 Angstroms. Again, for other processes, materialsother than an aluminum alloy may be more suitable. For example, dopedpolycrystalline silicon is also well-known as an effective conductiveinterconnect/gate material.

In FIG. 20, a resist layer 62 is formed on the metal layer 60 in anyconventional manner to a thickness of preferably about 2000 to 15000Angstroms and followed preferably by a photolithography process toresult in metal line pattern 64A.

In FIG. 21, an intra-metal spacing 64B is formed by etching metal layer60 using a conventional metal etching process to form an interconnectstructure consisting of patterned metal layer 60 and spacings 64B.Again, the particular etch chemistry and technique will depend on theparticular material selected for layer 60.

In FIG. 22, the metal interconnect structure of FIG. 21 is shown inconjunction with several other metal plugs 58, each capped by a sectionof metal interconnect layer 60. It will be understood, of course, thatit is not necessary to locate every interconnect line above a metalplug.

In a preferred first processing option, any material in spacings 64B isremoved and etched down to the surface of the device layer 50 with theresist layer 62 intact to form trenches 64C. As explained in connectionwith FIG. 11 as well, the depth of trenches 64C is adjustable and can bemade down to and even below the upper surface of the device layer 10(not shown in FIG. 22).

In a second processing variation of this embodiment (shown in FIG. 23),resist layer 62 is first stripped and previously etched metal layer 60is used as a hard mask to etch trenches 64C. The choice between thesetwo variations can be made on a case by case basis in accordance withconventional and well-known process requirements.

In FIG. 24 air gaps are formed in substantially the same manner asdepicted earlier for FIG. 12. That is, a silicon-dioxide or the likedielectric layer 66 is deposited onto the surface to a thickness ofabout 2000 to 10000 Angstroms. Poor step coverage by the deposition ofdielectric layer 66 results in the formation of intra-metal line airgaps 68. Air gaps 68 provide superior electric isolation due to the lowdielectric constant of air as previously discussed.

It will be appreciated by those skilled in the art that this secondembodiment can also be used to create structures that are similar tothose already illustrated in FIGS. 12A and 12B, including air gaps ofdifferent height, width, etc. Moreover, the above steps can be sequencedagain to form multi-level interconnect structures in the same manner aspreviously described for FIG. 13. Thus, air gaps can be used as aninsulation layer between inter-metal or intra-metal layers formed of Al,Al alloys, polycrystalline silicon, etc.

Third Embodiment

Referring generally now to FIGS. 25-33, a third embodiment of a methodfor forming an integrated circuit device having at least an air gapstructure is shown for a conventional metal interconnect structure ofthe type having aluminum alloy metal interconnect layers and aluminumalloy metal plugs. The primary difference to the second embodiment is inthe use of a different type of a barrier metal layer for the interlayerplugs.

In FIG. 25, a contact/via dielectric layer 14 is formed on device layer10 as before.

In FIG. 26, a photoresist layer 22 is formed and patterned on dielectriclayer 14 as before to form a pattern of openings 86A.

In FIG. 27, openings 86B are etched into contact/via dielectric layer 14as before.

In FIG. 28, resist layer 22 is stripped and an aluminum barrier layer 94(such as Ti/TiN, Ta, TaN or Aluminum oxide) is deposited on the surfaceof dielectric layer 82 and in openings 86. Again, these are merelyexamples of those known at this time to be particularly suited forAluminum, and other compositions may be needed for other types ofconductive line metals. For some metals, of course, a barrier layer maynot be needed in the first place.

An aluminum alloy layer 90 (preferably Aluminum with some smallpercentage of Cu and/or Si) is then deposited to a thickness of about500 to 8000 Angstroms, which completely fills contact/via openings 86Band provides an aluminum alloy interconnect layer coupled to aluminumalloy plugs 88.

This embodiment, therefore, is distinguished from the second embodimentnoted earlier in that the plug and interconnect layer can be formed in asingle step, thus improving throughput for those applications where itis acceptable to use something other than a Tungsten based plug.

In FIG. 29, as before a resist layer 92 is formed on the metal layer toa thickness of about 2000 to 15000 Angstroms followed by aphotolithography process.

In FIG. 30, an intra-metal spacing 74B is formed by etching the aluminummetal layer 90 using a conventional metal etching process as notedearlier for FIG. 21.

In FIG. 31, the metal interconnect structure of FIG. 30 is shown inconjunction with several other metal plugs 88, each capped by a sectionof aluminum alloy metal interconnect layer 90. As before, it will beunderstood, of course, that it is not necessary to locate everyinterconnect line above a metal plug.

In a preferred first processing option, any material in intra-metalspacings 74B is removed and etched down to the surface of the devicelayer 10 with the resist layer 92 intact to form trenches 74C. Asexplained in connection with FIG. 11 as well, the depth of trenches 74Cis adjustable and can be made down to and even below the upper surfaceof the device layer 10 (not shown in FIG. 31).

In a second processing variation shown in FIG. 32, resist layer 92 isfirst stripped and previously etched metal layer 90 is used as a hardmask to etch trenches 74. Again the choice between these two variationscan be made on a case by case basis in accordance with conventional andwell-known process requirements.

In FIG. 33 air gaps are formed in substantially the same manner asdepicted earlier for FIG. 12. That is, a silicon-dioxide dielectriclayer or the like 78 is deposited to fill the trenches 74 and cover themetal pattern 90 to a thickness of about 1000 to 8000 Angstroms. Poorstep coverage by the deposition of dielectric layer 78 results in theformation of intra-metal line air gaps 76. Air gaps 76 provide superiorelectric isolation due to the low dielectric constant of air aspreviously discussed.

It will be appreciated by those skilled in the art that this thirdembodiment can also be used to create structures that are similar tothose already illustrated in FIGS. 12A and 12B, including air gaps ofdifferent height, width, etc.

Moreover, the above steps can be sequenced again to form multi-levelinterconnect structures in the same manner as previously described forFIG. 13, and as shown generally in FIG. 34.

In FIG. 34, a composite drawing is shown of two metal interconnectstructures fabricated according to the third (and second) air gap methodof the present invention. In a first level of interconnect structureaccording to the present invention, metal interconnect 60, metal plugs58, and air gaps 68 are shown embedded in dielectric layer 14 abovedevice layer 10.

Note that as with FIG. 13, the resulting structure of FIG. 34 shows thatthat air gaps 68 can extend below the level of the upper surface ofdevice layer 50 and above the upper surface of metal lines 60, thusproviding maximum electrical isolation. Also shown in FIG. 34 is asecond level of metal interconnect layer 80 that includes an additionalmetal level 86, metal plugs 84, and air gaps 88. Air gap 88 providesintra-level metal isolation and extends to layers 50, 52, and 60.

Fourth Embodiment

A fourth embodiment is now described with reference to FIGS. 35-38. Thisembodiment is a variant of the first embodiment in which an etch stoplayer between a line dielectric and a via dielectric is eliminated tofurther reduce the effective dielectric constant of the inter-metaldielectric layer.

Thus, in FIG. 35, the second etch stop layer 16 between the via and linedielectric layers 14 and 18 (FIG. 2) has been eliminated to furtherreduce the effective dielectric constant of the inter-metal dielectriclayer. In lieu of two dielectric layers separated by an etch stop layer,a single dielectric layer 15 is deposited onto the surface of etch stoplayer 12. The single dielectric layer 15 is also ideally silicon dioxideor the like and is deposited to a thickness of about 1000 to 10000Angstroms in a manner similar to that already described for viadielectric layer 14.

An etch stop and/or anti-reflecting layer 20 is subsequently depositedon the line dielectric layer 15 as discussed before in connection withFIG. 2.

In FIG. 36, a photoresist layer 22 is formed on etch stop and/oranti-reflecting layer 20 to a thickness of about 1000 to 10000 Angstromsas already described in FIG. 3.

In FIG. 37, using metal contact or via pattern 24A as a mask, thedielectric layer 15 is anisotropically etched to form metal contact orvia opening 24B as already described in FIG. 4. The primary differencefrom FIG. 4 is that, as generally illustrated, dielectric layer 15 isonly partially etched, in this case, to a depth of approximatelyslightly more than half the thickness of such layer.

In FIG. 38, after removing the resist layer 22, another photoresistlayer 32 is processed to form metal line pattern 24C by photolithographyprocesses as generally already described in FIG. 5.

From this point forward, processing takes place in substantially thesame fashion as already illustrated above in connection with FIGS. 6-13,thus resulting in a single or multi-level air gap interconnectstructure, albeit with slightly modified layer compositions as notedhere.

Fifth Embodiment

A fifth embodiment is now described with reference to FIGS. 39-40. Thisembodiment is also a variant of the first embodiment in which a firstetching operation is performed only as far as a first etch stop layer.

Accordingly, FIG. 39 illustrates a variation in which given thestructure shown in FIG. 3, an etching operation is conducted in asimilar fashion to that already describe in FIG. 4, except that suchetch is stopped upon reaching second etch stop layer 16. In all otherrespects, this operation is the same, in that line dielectric layer 18is anisotropically etched using via pattern 24A as a mask to form ametal contact or via opening 24B. It is only the case, therefore, thatthese openings do not extend as far down as those illustrated in FIG. 4.

In FIG. 40, after removing the resist layer 22, another photoresistlayer 32 is formed. A subsequent etch transfers the upper profile ofopening 24C to the bottom of the openings, so that a deeper enlargedopening 24D results that is substantially the same as shown in FIG. 6.

From this point forward, processing takes place in substantially thesame fashion as already illustrated above in connection with FIGS. 7-13,thus resulting in a single or multi-level air gap interconnectstructure, albeit with slightly modified layer compositions as notedhere.

While there have been described above the principles of the presentinvention in conjunction with specific circuit implementations andapplications it is to be clearly understood that the foregoingdescription is made only by way of example and not as a limitation tothe scope of the invention. Particularly, it is recognized that theteachings of the foregoing disclosure will suggest other modificationsto those persons skilled in the relevant art. Such modifications mayinvolve other features which are already known and which may be usedinstead of or in addition to features already described herein. Althoughclaims have been formulated in this application to particularcombinations of features, it should be understood that the scope of thedisclosure herein also includes any novel feature or any novelcombination of features disclosed either explicitly or implicitly or anygeneralization or modification thereof which would be apparent topersons skilled in the relevant art, whether or not such relates to thesame invention as presently claimed in any claim and whether or not itmitigates any or all of the same technical problems as confronted by thepresent invention. The applicants hereby reserve the right to formulatenew claims to such features and/or combinations of such features duringthe prosecution of the present application or of any further applicationderived therefrom.

1. A method for forming an air gap structure in an integrated circuit,the method comprising: forming a device layer; and forming at least afirst dielectric layer on the device layer; and patterning said at leastfirst dielectric layer to form first openings therein; and forming atleast an aluminum based conductive layer on the at least firstdielectric layer and so as to fill said first openings therein; andremoving a part of said at least an aluminum based conductive layer toform an interconnect structure of aluminum based conductive lines; andforming a second opening between two adjacent aluminum based conductivelines in said interconnect structure; and wherein said second opening ischaracterized by a trench depth that can be varied by controlling anetch process; forming a second dielectric layer over said second openingto form at least one air gap between said two adjacent aluminum basedconductive lines; wherein said second dielectric layer includes both aCVD layer, and an HDPCVD layer formed in sequence; further wherein saidat least one air gap has a height that is controlled in part by saidtrench depth.
 2. The method of claim 1, wherein said trench depth canextend below respective bottom portions of said two adjacent aluminumbased conductive lines so as to effectuate a desired air gap size. 3.The method of claim 1, wherein said air gap height is further controlledby controlling parameters of a deposition operation used for formingsaid second dielectric layer, including a step coverage parameter. 4.The method of claim 1, wherein said trench depth is controlled bycontrolling a time of an etch process.
 5. The method of claim 1, whereinsaid two adjacent aluminum based conductive lines each include a lineportion and a pillar portion underneath the line portion, and said atleast first dielectric layer extends from said pillar portion tounderneath said line portion.
 6. The method of claim 1 wherein said atleast one said air gap has a height above said two adjacent aluminumbased conductive lines.
 7. The method of claim 5, wherein said at leastone said air gap has a depth below said pillar portion.
 8. The method ofclaim 1, further including using the aluminum based conductive structureas a mask to form said second opening.
 9. The method of claim 1, whereinsaid aluminum based conductive layer includes an aluminum alloy.
 10. Themethod of claim 1, wherein said aluminum based conductive layer includesa refractory material barrier layer.
 11. A method for forming an air gapstructure in an integrated circuit, the method comprising: forming adevice layer; and forming at least a first dielectric layer on thedevice layer; and patterning said at least first dielectric layer toform first openings therein; and forming at least a first aluminum basedconductive layer on the at least first dielectric layer and so as tofill said first openings therein; and removing a part of said at least afirst aluminum based conductive layer to form a first aluminum basedinterconnect structure of first aluminum based conductive lines; andforming at least a second dielectric layer on the first aluminum basedinterconnect structure of first aluminum based conductive lines; formingsecond openings in said second dielectric layer; and forming a secondaluminum based interconnect structure of second aluminum basedconductive lines on said at least second dielectric layer, said secondaluminum based interconnect structure also filling said second openingsto form contacts to said first aluminum based interconnect structure;forming at least one third opening which extends from said secondaluminum based interconnect structure to said first aluminum basedinterconnect structure; and forming a third dielectric layer over saidthird opening to form at least one multi-level air gap that is situatedbetween adjacently located aluminum based conductive lines in saidsecond aluminum based interconnect structure of second aluminum basedconductive lines; wherein said at least one multi-level air gap operatesto isolate adjacently located aluminum based interconnect conductivelines in at least two separate aluminum based interconnect levelslocated at different distances from said device layer, including in saidfirst aluminum based interconnect structure and said second aluminumbased interconnect structure.
 12. The method of claim 11, furtherincluding steps of forming a first single level air gap in said firstaluminum based interconnect structure, and forming a second single levelair gap in said second aluminum based interconnect structure.
 13. Themethod of claim 11, further including a step of forming an air gapdirectly on top of at least one of said first aluminum based conductivelines.
 14. The method of claim 11, wherein said multi-level air gap isformed using said first aluminum based conductive lines and said secondaluminum based conductive lines as a mask, and without using aphotoresist mask.
 15. The method of claim 11, wherein said at least onemulti-level air gap extends below a top surface of said device layer.16. The method of claim 11, wherein said at least one multi-level airgap extends above a top surface of said aluminum based conductive layer.